In recent years, laser beams have become more and more important as an universal tool for material processing. Particularly, laser beams of a CO2-Laser having a laser power in the order of one to several kW are used in manufacturing processes like welding, cutting, drilling, soldering, inscribing, for surface treatment and so on. In these processes, it is important that the light intensity distribution and the position of the laser beam are monitored even during the actual material processing. Within the present d description, monitoring the position of a laser beam which is a special case of monitoring the light intensity distribution over the cross-section of the laser beam will not always be especially highlighted, because a change in position is nothing else than a movement of the center and the borders of the light intensity distribution of the laser beam.
From U.S. Pat. No. 5,694,209 it is known to scan the cross-section of a laser beam with a small reflecting element and to analyze a laser light reflected out of the laser beam with the reflecting element. The mechanical efforts which are required for scanning the laser beam with the reflecting element are enormous. Additionally, the beam quality of the laser beam is decreased due to scanning it with the reflecting element. If the laser beam is a laser beam of a CO2-laser, for example, which has a wave length in the infrared range, the analysis of the light intensities reflected out of the laser beam is also difficult due to the general difficulties in determining small intensities of light particularly in the infrared range.
For analysing or monitoring a light intensity distribution over a cross-section of a laser beam, it is known from German Patent Application published as DE 40 06 618 A1 to provide a laser mirror reflecting the laser beam with small channels. If these channels have a defined geometry and defined distances, a portion of the laser beam incident on the channels leaves the surface of the laser mirror as a measurement radiation whose partial waves starting at the channels are superimposed in an intensifying way in at least one direction. Thus a measurement beam is formed which is detected by a detector. The portion of the laser beam incident on the plane surface between the channels is geometrically-optically reflected as a main beam at the same time. If the total area of all channels is small as compared to the total mirroring surface, only a very limited portion of the light intensity of the laser beam is coupled out towards the detector. The actual loss in laser power, however, is relevant. Additionally, there is the effort for forming the laser mirror in the special way, and there are difficulties in analyzing the small out-coupled laser light intensities with regard to their spatial distribution, particularly, if it is infrared laser light of the laser beam of a CO2-laser, for example.
As a reference to its own prior art DE 40 06 618 A1 also describes a method of analysing or monitoring a light intensity distribution over a cross-section of a laser beam in which a residual roughness of a plane mirroring surface of a laser mirror reflecting the laser beam is used for obtaining scattered light which is detected with a detector. The roughness of presently used laser mirrors, however, is typically only just 1/1000 of the laser wave length. Thus, present laser mirrors have a residual roughness which is far below the value shown by laser mirrors 10 to 20 years ago, i.e. at the application date of DE 40 06 618 A1. At that time, a residual roughness of laser mirrors for laser beams of 1/10 to 1/20 of the laser wave length was common. Therefore, in contrast to the past, there should be no relevant scatter signal left, if a present laser mirror is considered. Even if such a scatter signal would still be there, it would be in the infrared range in case of infrared laser beams, and thus it could only be analyzed with corresponding high efforts.
Thus, there is still a need for a method and a device for analysing and/or monitoring the light intensity distribution over the cross-section of a laser beam, which may even then be realized with low constructive efforts, if the light intensity distribution of an infrared laser light beam is to be detected, and which at the same time avoid any relevant deformation of the laser beam over its cross-section.